1. Field of the Invention
The present invention relates to a method for producing a soft magnetic material which comprises sintering a press-molded soft magnetic powder.
2. Description of the Related Art
In recent years, methods for producing a soft magnetic material which comprises sintering a press-molded soft magnetic powder for the purpose of an increase in magnetic permeability, a decrease in iron loss, and the like, of a soft magnetic material, have been studied. As shown in Japanese Unexamined Patent Publication No. 5-36514, these methods comprise first oxidizing the surfaces of an atomized alloy powder in air to form a soft magnetic Ni—Zn ferrite thin film on the surfaces of the powder, subsequently sputtering Al in a nitrogen ambient atmosphere to form an AlN base insulating film on the Ni—Zn ferrite thin film, adding a B2O3 powder to this soft magnetic powder to prepare a molding compound of a soft magnetic material, press-molding the molding compound into a predetermined shape, and then sintering the press-molded soft magnetic powder at 1,000 degrees centigrade under pressure, by hot pressing, to produce a soft magnetic material.
However, the above-mentioned method has a problem in that, when sintering the press-molded soft magnetic powder under pressure by hot pressing, the insulating film on the surface of the soft magnetic powder is cracked by the press pressure, and the insulation among soft magnetic powder particles deteriorates, and then the iron loss (eddy current loss) of the sintered soft magnetic material increases. If a thick insulating film is formed in order to prevent the insulating film from cracking, the density of a magnetic material in the soft magnetic material will deteriorate, the saturation magnetic flux density will deteriorate, and the magnetic property will worsen. Moreover, the hot press sintering has drawbacks in that the bonding strength among soft magnetic powder particles is weak and the mechanical strength of a soft magnetic material is weak. Furthermore, it has also problems that the step of forming a soft magnetic Ni—Zn ferrite thin film on the surface of an atomized alloy powder and the step of sputtering Al in a nitrogen ambient atmosphere to form an insulating film take time and effort and, then, the production cost becomes high.